The major purpose of this investigation is to understand the mechanism by which cell interactions specify embryonic cells to particular fates and patterns during embryonic induction. We have developed a model system to define the roles that axial embryonic structures, the neural tube and notochord, have in committing or inducing mesodermal cells of the somite to a myogenic fate. In the culture system that constitutes the model, we remove somites from early embryos and culture them as explants either alone, recombined with a segment of neural tube or notochord, or across a filter from neural tube or notochord. Myogenesis is assessed by expression of fast and slow isoforms of myosin heavy chains and expression of myogenic regulatory factors. We report data showing that the neural tube and the notochord independently induce myogenesis in explanted unspecified somites (somites which when cultured alone do not form muscle fibers). We also find that myogenesis can be induced by neural tube in segmental plate explants as well, demonstrating induction of myogenesis in mesodermal cells before they become part of either somites or myotomes. The activity responsible for these inductions is confined principally to the ventral half of the neural tube or the notochord, and we show that it is active across a filter in inducing myogenesis in unspecified somites or segmental plate. Thus the process is controlled by a diffusible factor(s). The dorsal neural tube, on the other hand, contains a factor that blocks differentiation of cells already committed to myogenesis. While a number of growth factors were tested in this model, only TGFb1 was active in inducing myogenesis in unspecified somites or segmental plate explants that otherwise do not form muscle fibers. Using RT-PCR, we find that the cells of segmental plate and unspecified somites, even though they are unable to differentiate into muscle when cultured along, express mRNA for the avian myogenic regulatory factor, MyoD1. We also find that the neural tube expresses mRNA for members of the TGFb growth factor superfamily. Our hypothesis is that paraxial mesoderm cells become permissive of myogenesis (i.e., transcription of the myogenic regulatory genes begins) prior to segmentation of the somites. Under the influence of diffusible factors from the neural tube and/or the notochord, these cells become specified to a myogenic fate. Interaction between inhibitory influences from the dorsal neural tube and positive influences from the ventral neural tube patterns the formation of the myotome of the developing somite. The aims of this proposal are: to identify the cells of the neural tube responsible for the induction, to identify the role of myogenic regulatory factors in the mechanism of induction, and to identify the factors in the ventral and dorsal neural tube that foster or inhibit myogenesis.